Electrolytic cell with in-situ charging electrolyte

ABSTRACT

A silver coulometer using a standard phosphoric acid - silver phosphate electrolyte has a metaphosphate added thereto to permit the in situ charging of the coulometer. The resultant silver deposit is smooth, bright, adherent, economic and more accurate than prior art deposits for silver coulometers.

O Unlted States Patent [151 3,684,928 Roethlein [451 Aug. 15, 1972 [54] ELECTROLYTIC CELL WITH IN-SITU 3,052,830 9/1962 Ovshinsky ..317/231 CHARGING ELECTROLYTE 3,423,643 1/1969 Miller ..317/231 3,423,648 1/1969 Mintz ..317/231 [72] Invent f Bennmgmn 3,564,347 2/1971 Peck ..317/230 [73] Assignee: Sprague Electric Company, North Primary Examiner-James Kallam Ada M Attorney-Connolly & Hutz and Vincent l-l. Sweeney [21] Appl' 149984 A silver coulometer using a standard phosphoric acid silver phosphate electrolyte has a metaphosphate [52] 0.8. CI. ..317/230, 317/231, 252/622 added thereto t p rmit th in situ charging of the [51] Int. Cl. ..H0lg 9/02 cculometer- The resultant Silver deposit is smooth. [58] Field of Search ..317/230, 231, 233 bright, adh economic and m r a urate than prior art deposits for silver coulometers. [56] Rem-wees Cited. 7 Claim, 2 Drawing figures UNITED STATES PATENTS 2,791,473 5/l957 Mattox ..340/213 2 17 E5 28 5 E E:

ELECTROLYTIC CELL WITH IN-SITU CHARGING ELECTROLYTE BACKGROUND OF THE INVENTION This invention relates to electrolytic cells with an in situ charging electrolyte, and more particularly to plating additives for electrolytes for low charge density silver coulometers.

An electrochemical timing device which employs the principles of a silver coulometer activates a relay by a voltage rise occurring within the coulometer cell when a predetermined quantity of silver has been removed from the positive (anode) electrode. The voltage rise is caused by a shift in potential on the inert anode electrode, gold to its next highest electrode process, oxygen evolution. The initial quantity of silver on the anode along with the discharge current, therefore, determines the running time of the device.

The prior art teaches that anode-charging has been done in a separate plating bath (silver cyanide or the like), since plating in the silver phosphate/phosphoric acid electrolyte produces rough granular deposits with poor adherence. A cyanide bath produces smooth, bright deposits, but does have the disadvantage of redissolving some of the electro-deposited silver. Another disadvantage of depositing the silver in a separate plating bath, is that when the deposited silver is then placed into the timer device, there is always the danger of knocking off some of the silver. It would also be much more economical to deposit the silver in situ rather than in a separate plating bath prior to transferring same to the device to be used.

Accordingly, it is an object of the present invention to provide an economical and accurate system for plating silver on a gold substrate for subsequent use in an electrochemical timing device.

It is another object of this invention to provide an electrolyte plating additive that will allow this plating to take place, in situ, within the device itself, and will produce smooth silver deposits with good adherence.

SUMMARY OF THE INVENTION A standard silver coulometer electrolyte of silver phosphate and phosphoric acid contains a relatively small amount (about 0.01M) of a metaphosphate added thereto. This combination increases the overvoltage of the system, decreases the rate of surface diffusion of the silver adions, and permits the in situ charging of silver from the cathode electrode to a gold anode within the coulometer. The resultant silver deposit is smooth, bright, adherent, economic and more accurate than prior art deposits for silver coulometers.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view in section of an electrochemical timer with the electrolyte of the present invention; and

FIG. 2 is an enlarged view of region 28 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT It has been found that certain plating additives when added in small quantities (about 0.01 molar) to a standard phosphoric acid silver phosphate coulometer electrolyte enhance a silver deposit from cathode to anode to a considerable degree when used in conjunction with a silver coulometer. This is especially true of low charge density silver coulometers. These plating additives provide the additional benefit of increased adhesion between the silver deposit and a gold substrate. The addition of these compounds to the plating electrolyte provides a more suitable means of charging a silver coulometer in situ, from within the silver container an operation that is more economical and more accurate than a separate plating bath.

The electrolyte containing the metaphosphate additive can advantageously be used in an electrochemical timer device such as is described in US. Letters Pat.

- No. 3,564,347 issued on Feb. 16, 1971, and assigned to the same assignee as is involved herein. Such a device is best described by reference to the drawings.

FIG. 1 of the drawings shows an electrochemical timer 10 that has a silver cathode can 12. The cathode can 12 has a metal cathode lead 14 affixed thereto. Located within can 12 is an insulating bobbin 16 having a threaded central shaft 17. Located down through the center of bobbin l6 and helically wound onto threaded central shaft 17 of said bobbin is a gold or other metal anode wire 18 which is to be plated with silver from the cathode can 12. Alternatively the metal anode could be in the form of a thin metal film instead of a wound wire. Placed in contact with the anode and cathode is an electrolyte 20. Bobbin 16 is provided with end shoulders 19 which fit against the inside surface of cathode container 12. A metal anode lead 22 is in contact with anode wire 18 at point 24. Located about the juncture of anode lead 22 and anode wire 18, and closing can 12, is a resilient sealing bung 26. That part of anode wire 18 in the center of bobbin 16 is not exposed to the electrolyte, and, thus, is not part of the active anode of the timer. Region 28 of FIG. 1 is shown enlarged in FIG. 2. This Figure illustrates that electrolyte 20 is available in the trough 30 of the threaded central shaft 17. The electrolyte used herein is composed of approximately IN Ag PQ, and 62 weight percent of H PO The insulating, non-conductive bobbin 16 can be made of either organic or inorganic material, any suitable organic polymer, ceramic or other inorganic material. The end shoulders 19 thereof should be in engagement with the side of the silver can 12, and the bobbin thereby should have a surface extending in substantial parallelism with a portion of the wall of the can 12. Speaking more generally, the electrolyte described herein will enhance the in situ charging of any silver cathode onto a metal anode. The silver cathode is most advantageously the container itself, but any silver cathode may be used with this electrolyte.

Prior art techniques have plated silver onto the metal anode wire 18 prior to inserting it into the cathode can 12 because silver deposits produced in situ therein had very large granules and therefore, much less accuracy when used in electrochemical timer devices. However, it has been discovered that this silver plating of the metal anode wire 18 can advantageously be carried out in situ directly within the electrochemical timer 10 when at least 0.001M metaphosphate in the form of metaphosphoric acid or sodium metaphosphate is added to the phosphoric acid silver phosphate electrolyte currently being used with such devices. The most advantageous results are produced when the concentration of the metaphosphate added is between I O.lM(molar) and 0.01M, and the silver deposits produced herein will have good adherence to a gold anode wire or a gold film. It should be noted that other conductive materials may be used as the anode wire or electrode, and these include carbon, boron carbide, iridium, rhodium, palladium and platinum.

With no intention of being bound by any theory, it is believed that adsorption of metaphosphate takes place on low energy sites of the gold substrate in a manner which blocks those sites, and silver deposition is then forced to higher energy sites with a subsequent increase in overvoltage. As coverage increases the overvoltage rises so that the activation energy needed for nucleus formation is surpassed, and the rate of surface diffusion of the silver adions is diminished, and two dimensional nucleation of the silver adions becomes a part of the overall electrode process. Silver adions condense together without random-walking separately to lattice building sites, and two dimensional nucleation produces smaller grained deposits and, subsequently, smoother silver deposits having good adherence. And, of course, a smoother silver deposit will produce a more accurate timer device.

Electron microscopy photos were taken of electrodeposited silver surfaces under varying conditions. Samples plated in the absence of metaphosphate had a granular structure with well defined growth of crystal faces; a lower silver ion concentration still retained the large grain structure but with less individual crystal definition. Growth in the presence of metaphosphate produced a smooth structure, consisting of very fine needles in the presence of a 1.0 normaHN) silver ion concentration. At a lower silver ion concentration the grains had little texture. The silver crystals produced in the presence of metaphosphate were much smaller and finer than those produced in the absence thereof. By comparison, crystals from a metaphosphate environment measured approximately l-2 microns in diameter, while those produced in the absence of metaphosphate measured approximately 6-10 microns. Consequently, the metaphosphate influenced silver deposits were much smoother than the others.

While a metaphosphate is discussed herein, as being an advantageous electrolytic additive, other compounds or additives that can be used to produce similar, although somewhat less desirable, results when used in conjunction with a phosphoric acid silver phosphate electrolyte includes succinic acid, lactic acid, sodium acetate, glycine and tartaric acid. All of these additives will produce smoother silver deposits when added to the phosphoric acid silver phosphate electrolyte than in their absence. Tartaric acid, for example, is an effective addition agent for producing smooth silver deposits by in situ charging, but has the disadvantage of lowering the stop voltage therein below the required level. This disadvantage, however, is

amenable by effective concentration changes.

Most advantageous results are obtained where the electrolyte is approximately 62 weight percent phosphoric acid solution containing approximately LON Ag obtained from Ag PO and about 0.01M metaphosphate added to permit the in situ charging of the timer device by producing smooth silver deposits However, an e le trolyge having 0.0lN t 2N 111 a solution 0 p osp one act rangmg mm weight percent to 85 weight percent would also be favorably affected by the addition of the small amounts of the plating additives of this invention. The electrolyte of the present invention as described above operates from temperatures below 55 to above 75 said electrolyte; said electrolyte comprising a composition of a silver salt in a solution of phosphoric acid, and having a plating additive of a metaphosphate added thereto.

2. The in situ charged electrolytic cell of claim 1 wherein said silver salt is a silver phosphate of between 0.0lN (normal) to 2.0N in a solution of phosphoric acid ranging from 30 weight percent to weight percent, and wherein said plating additive is present therein in a concentration of between 0.1M and 0.001M.

3. The in situ charged electrolytic cell of claim 2 wherein said silver salt is a silver phosphate of approximately lN in a solution of approximately 62 weight percent of phosphoric acid, and wherein said plating additive is approximately 0.01 M metaphosphate.

4. The in situ charged electrolytic cell of claim 2 wherein said metaphosphate is in the form of sodium metaphosphate or meta-phosphoric acid.

5. The in situ charged electrolytic cell of claim 2 wherein said silver cathode electrode is in the form of the cell container, said electrolyte being in said container.

6. The in situ charged electrolytic cell of claim 5 wherein said metal anode electrode is at least one conductive material selected from the group consisting of carbon, boron carbide, iridium, rhodium, palladium, platinum and gold.

7. The in situ charged electrolytic cell of claim 6 wherein said metal anode electrode is a gold wire or a thin film of gold. 

2. The in situ charged electrolytic cell of claim 1 wherein said silver salt Is a silver phosphate of between 0.01N (normal) to 2.0N in a solution of phosphoric acid ranging from 30 weight percent to 85 weight percent, and wherein said plating additive is present therein in a concentration of between 0.1M and 0.001M.
 3. The in situ charged electrolytic cell of claim 2 wherein said silver salt is a silver phosphate of approximately 1N in a solution of approximately 62 weight percent of phosphoric acid, and wherein said plating additive is approximately 0.01M metaphosphate.
 4. The in situ charged electrolytic cell of claim 2 wherein said metaphosphate is in the form of sodium metaphosphate or meta-phosphoric acid.
 5. The in situ charged electrolytic cell of claim 2 wherein said silver cathode electrode is in the form of the cell container, said electrolyte being in said container.
 6. The in situ charged electrolytic cell of claim 5 wherein said metal anode electrode is at least one conductive material selected from the group consisting of carbon, boron carbide, iridium, rhodium, palladium, platinum and gold.
 7. The in situ charged electrolytic cell of claim 6 wherein said metal anode electrode is a gold wire or a thin film of gold. 